1. Field of the Invention
The present invention relates to the use of perfluoropolyethers in mechanical pumps for the generation of vacuums .ltoreq.5.times.10.sup.-4 torr.
2. The Prior Art
The use of perfluoropolyethers in the vacuum pumps is well known, due to the high stability of these compounds which permits their viscosity variation to be sufficiently low during the operation of the pumps.
The perfluoropolyethers are the best fluid to be used in said vacuum apparatuses as compared with the conventional fluids, such as mineral and silicone oils, since the latter tend to decompose owing to both the heat generated by friction forces during the pump run, and to the chemical action of the substances entering the pump during its run. However, also the perfluoropolyethers utilized at present in the vacuum technique do not enable to reach high vacuums, i.e. lower than or equal to 5.times.10.sup.-4 torr.
The vacuum usually obtained with said perfluoropolyethers is of the order of 10.sup.-3 torr.
The mechanical pumps are generally used in the field of microelectronics, in particular in that of semiconductors.
In these fields, the composition of the residual atmosphere in the vacuum chamber where the semiconductor processing takes place is critical.
It has been observed that the perfluoropolyethers utilized at present do not permit to obtain residual atmospheres with a sufficiently low pollution degree, as is required for the electronics processing. For this reason, the residual impurities cause a considerable reduction of the number of utilizable semiconductor pieces.